A positive displacement expander such as a rotary expander is conventionally known as an expander which serves to generate power when a high-pressure fluid expands (for example, refer to patent document 1). Such expander is used for an expansion process of a vapor compression type refrigeration cycle (for example, refer to patent document 2).
The above-mentioned expander comprises a cylinder and a piston revolving along the inner circumference of the cylinder, and an expansion chamber formed between the cylinder and the piston is partitioned into the suction/expansion side and the discharge side. As the piston revolves, the suction/expansion side in the expansion chamber is changed into the discharge side, and the discharge side is changed into the suction/expansion side alternately, thus the suction/expansion action and the discharge action of the high-pressure fluid are concurrently and collaterally carried out. In this manner, the expander recovers the rotation power generated due to expansion of the fluid in order to utilize the rotation power as, for example, a drive source of a compressor.
An expansion ratio, the density ratio of the suction fluid to the discharge fluid is predetermined as a design expansion ratio for the above-mentioned expander. The design expansion ratio is determined on the basis of the pressure ratio of the high-pressure to the low-pressure in a vapor compression type refrigeration cycle that is carried out using the expander.
In the actual operation, however, since the temperature subject to cooling or the temperature subject to radiation (heating) vary, the above-mentioned pressure ratio of the refrigeration cycle may become smaller than that assumed in the design phase. Specifically, when the low-pressure in the vapor compression type refrigeration cycle rises, the pressure of the fluid expanded may be lowered than the above-mentioned low-pressure in the design expansion ratio (herein after referred to as expansion pressure). In this case, since the expander excessively expands the fluid, the pressure of the fluid dropped to the above-mentioned expansion pressure is raised once up to the above-mentioned low-pressure before discharging the fluid. Accordingly, a workload which results when the fluid is excessively expanded by the expander, and extra power for discharging the fluid having increased pressure may be consumed. Thus, an expander capable of reducing overexpansion loss yielded due to such reasons has been conventionally desired. To solve such problems, the applicant of the present application devised an expander which by passes part of the fluid on the inflow side (high-pressure fluid) of the expansion chamber to the suction/expansion process position. Specifically, the expander is equipped with a communication path for diverging from the inflow side of the fluid into the expansion chamber and communicating with the suction/expansion process position of the expansion chamber. The communication path is provided with an electric-operated valve as a circulation control mechanism for regulating a flow rate of the high-pressure fluid that is bypassed through the communication path.
In the expander of the above-mentioned configuration, for example, when the low-pressure in the refrigeration cycle is higher than the expansion pressure of the expander as mentioned above, the electric-operated valve is opened at a predetermined degree of opening, and the high-pressure fluid is bypassed through the communication path to the suction/expansion process position of the expansion chamber. Then by raising the expansion pressure of the expander close to the above-mentioned low-pressure, the above-mentioned overexpansion loss can be reduced (refer to patent document 3).    Patent document 1: Japanese Patent Application Publication No. 8-338356    Patent document 2: Japanese Patent Application Publication No. 2001-116371    Patent document 3: Japanese Patent Application Publication No. 2004-197640